1. Field of the Invention
This invention relates to an optical film and an antireflective film, and a polarizing plate and an image display device using the same.
2. Description of Related Art
With the recent progress toward large screen liquid crystal displays (LCDs), LCDs provided with optical films such as antireflective films or light diffusion sheets have been on the increase. In various image display units such as a liquid crystal display (LCD), a plasma display panel (PDP), an electro luminescence display (ELD), and a cathode-ray tube (CTR), for example, an antireflective film provided as the outermost face of the display in general to prevent lowering in contrast and extraneous images caused by the reflection of outside light. On the other hand, a light diffusion sheet is employed in the back light of a liquid crystal display.
An antireflective film, which is one of optical films, is usually constructed by laminating a light diffusion layer or a high refractive index layer and a low refractive index layer on a transparent support. To form these layers, it has been a common practice to form a thin metal oxide film by the chemical vapor deposition (CVD) method or the physical vapor deposition (PVD) method, in particular, the vacuum deposition method or the sputtering method, each falling within the category of the physical vapor deposition method, or by the coating method at a high productivity which comprises applying a coating solution for forming such a layer (JP-A-59-50401). Moreover, there have been proposed an antiglare film having a light diffusion layer containing light-transmitting particles (JP-A-11-305010, etc.) and a method of coating the surface with a hard layer (JP-A-2002-139602, etc.).
Because of being used as the outermost face of an image display device such as a display, an antireflective film should be free from surface errors, for example, irregularities and streaks. By using the technique reported by JP-A-11-305010, however, it is feared that the antiglare properties vary depending on the change in the partial film thickness arising in the course of forming the light diffusion layer, which brings about surface errors such as irregularities and streaks.
In the case of bonding an antireflective film to the surface of a display, it is further required that the antireflective film has a scratch resistance against small scuffs due to abrasion and such a high hardness as withstanding a strong writing pressure with a hard pen or pencil.
According to the technique of JP-A-2002-139602, however, the coating film hardness cannot be elevated to a sufficient level.
With the expansion of the display market, the tendency toward large screens and the progress into data broadcasting with digitalization in recent years, it is required to improve various properties other than the antiglare properties and scratch resistance of antireflective films. Among all, problems to be immediately solved as follows: 1) improving the contrast; 2) lessening blur characters; 3) establishing definitiveness in black color in black display; and 4) relieving dazzling in the case of being bonded to a large-scaled monitor having a moderate to low resolution of about 100 ppi. Thus, it is urgently required to develop an antireflective film that has satisfactory antiglare properties and fulfills the above requirements 1) to 4) at the same time.